1. Field of the Invention
The present invention relates to a communication system which distributes continuous stream packets from at least one server to multiple clients via an IP network.
Priority is claimed on Japanese Patent Application No. 2005-62293, filed Mar. 7, 2005, the content of which is incorporated herein by reference.
2. Description of Related Art
To implement a service such as video distribution, a multicast is defined in an open systems interconnection (OSI) Layer-3 internet protocol (IP), and various types of protocols corresponding thereto are implemented and executed (see e.g. Non-Patent Document 1: Beau Williamson, “IP Multicast Network Development Guide Vol. 1” Softbank Publishing Corp., Jul. 3, 2001.). Various types of highly reliable multicast communication protocols for supporting multicasts without losing data have been proposed (see e.g. Non-Patent Document 2: M. Handley et al., “The Reliable Multicast Design Space for Bulk Transfer” The Internet Society, 2000).
One general example of a packet loss compensation system in a multicast distribution network currently in active use is a forward error correction (FEC) based system, disclosed in Non-Patent Document 2. In this method, error correction information relating to one or a number of packets is transmitted simultaneously with the relay data, or multiplexed as appropriate, and errors are corrected on the receiving side. Non-Patent Document 2 also discloses various systems based on data retransmission in accordance with an acknowledgement (ACK) or a negative-acknowledgement (NACK) from the receiving side.
However, even if a high-reliability IP network is constructed using a router (L3 (Layer-3)) with a redundant configuration, a time of approximately several seconds to several tens of seconds is required for conventional multicast path control protocols due to dynamic changes in the multicast distribution path which comes with L3 switch/link disorders and the like. All data distributed during this period is lost.
In the FEC-based packet loss compensation system, a fixed rate of redundant data must be relayed irrespective of whether or not there are packet losses. This makes it impossible to compensate for a large amount of continuous packet loss of the type mentioned above. Moreover, retransmission-based systems require the addition of an ACK/NACK reply function on the terminal side, and the addition of a retransmission function to the router and the transmission terminal. Protocols which guarantee the arrival of all packets are problematic in that they do not guarantee the arrival times.
In view of the above problems, the present inventors have disclosed in Japanese Patent Application No. 2004-060528 a technique, used when implementing a broadcast-type multicast communication service using an IP network having a redundant configuration, for compensating for communication breaks caused when disorders in the IP network are detected and repaired. In Japanese Patent Application No. 2004-060528, the present inventors have disclosed a technique for achieving a higher level of compensation for communication breaks and for reducing the traffic on the network. There is also a demand to reduce traffic on networks on which there are plural multicasts.